1. Field of the Invention
This invention relates to control systems in which an instrument such as a key or card having a code recorded on it is submitted to a reader which reads the recorded code. Such control systems may be used in magnetic locks, and various accounting and credit control facilities. In the case of a magnetic locking system, the reader would control the operation of a lock and the coded instrument would be in the form of a key which could be inserted into the reader. In the case of a credit control system, the instrument would usually be in the form of a coded card.
2. Description of Prior Art
There are various known control systems of the above general type, as typified by the systems disclosed in U.S. Pat. Nos. 3,508,031, 3,210,527 and 3,590,333. In the system shown in U.S. Pat. No. 3,508,031, a card having several copper discs is inserted into a card reader which has primary and secondary windings disposed to opposite sides of a card sensing area. The secondary windings are connected to silicon controlled rectifiers. On insertion of a card into the reader, a switch is closed to apply a pulse to the primary windings which then generate magnetic fields. In response to those magnetic fields that are not obstructed by copper discs in the card, the corresponding secondary windings generate output signals which trigger response rectifiers whereby the code recorded on the card is identified.
U.S. Pat. No. 3,210,527 also describes an apparatus in which the reader has primary and secondary coils. The primary coils are energised to produce magnetic fields which are modified by the coded instrument to produce signals in the secondary windings whereby the coded information is sensed.
In the system described in U.S. Pat. No. 3,509,333, a card reading station includes a reading head which sweeps across a coded card in order to generate a sequence of electric impulses representative of the code recorded on the card.